A hydraulic control system for the transmission mechanism of an automatic power transmission for an automotive vehicle generally includes shift valves each operable between two gear ratio conditions to produce two prescribed forward drive gear ratios such as first and second or low and intermediate or second and third or intermediate and high gear ratios in a forward drive range in the power transmission system, as is well known in the art. Each of the shift valve thus incorporated into the hydraulic control system includes a valve spool which is alternately movable between predetermined axial positions respectively corresponding to the particular forward drive gear ratios allocated to the shift valve. The valve spool is constantly acted upon by a fluid pressure, usually called line pressure, continuously variable with the road speed of the vehicle installed with the transmission system and a fluid pressure, usually called throttle pressure representative of the load on the engine or the opening degree of the throttle valve which the engine has mounted in the carburetor thereof. The valve spool is thus movable between the above mentioned predetermined axial positions thereof depending upon the relationship between the governor and throttle pressures and the force of a spring opposing the governor pressure acting on the valve spool.
In an ordinary hydraulic transmission control system thus arranged basically, the valve spool of each shift valve is formed with a differential pressure acting area to be acted upon a suitable control fluid pressure such as the above mentioned throttle pressure or a line pressure developed by a pressure regulator valve also provided in the hydraulic control system. While the governor pressure as well as the force of the above mentioned spring is allowed to act on the valve spool of each shift valve irrespective of the axial position of the valve spool insofar as the governor pressure is available in the hydraulic control system, the control pressure to act on the differential pressure acting area of the valve spool is allowed into the shift valve when, and only when, the valve spool is held in the axial position producing the higher one of the gear ratios allocated to the shift valve. For this reason, the valve spool of each shift valve is caused to move from the lower gear ratio position to the higher gear ratio position depending only upon the relationship amongst the governor and throttle pressures and the force of the spring and from the higher gear ratio position to the lower gear ratio position depending upon the relationship amongst not only these three parameters but the control pressure acting on the differential pressure acting area of the valve spool. Thus, an upshift between the gear ratios allocated to each shift valve occurs when the governor pressure or the road speed of the vehicle are increased and/or the throttle pressure or the load on the engine is reduced as compared, respectively, with the throttle pressure or the load on the engine and/or the governor pressure or the vehicle speed in terms of a first predetermined relationship between the governor and throttle pressures or the vehicle speed and the engine load and, likewise, a downshift between the gear ratios takes place when the governor pressure or the vehicle speed is reduced and/or the throttle pressure or the engine load is increased as compared with the latter and former, respectively in terms of a second predetermined relationship therebetween. The differential pressure acting area of the valve spool is formed in such a manner that, when the differential pressure acting area is acted upon by the control pressure, the valve spool is urged to move toward the higher gear ratio position against the force resulting from the governor pressure acting on the valve spool by the sum of the force of the spring, the force resulting from the throttle pressure and the force resulting from the control pressure acting on the differential pressure acting area. If, therefore, the load on the engine is unchanged, the above mentioned first predetermined relationship occurs at vehicle speeds higher than those at which the second predetermined relationship occurs and, if the vehicle speed is unchanged, the first predetermined relationship occurs at engine loads less than those at which the second predetermined relationship occurs. The predetermined relationship above mentioned is ordinarily referred to as shift pattern. The difference between the shift patterns predominant over the upshift and downshift between the gear ratios allocated to a shift valve is dictated by the differential pressure acting area of the shift valve as will have been understood from the foregoing description.
Because, however, of the fact that both of the shift patterns predominant over the upshifting and downshifting actions of a shift valve are determined means of the shift valve per se, there arises a problem each of the shift pattern can not be made satisfactory without sacrificing the other although it may be possible to obtain a few satisfactory shift points on the shift pattern which is sacrificed.